Conventionally, an image forming apparatus is known as a quadruple tandem type color copying machine in which four photoconductive drums are arranged in parallel in a horizontal direction. This kind of color copying machine is equipped with a single polygon mirror (optical scanner) which is arranged between the second photoconductive drum and the third photoconductive drum. Further, the color copying machine is equipped with four light sources for forming respective electrostatic latent images on the surfaces of the respective photoconductive drums. The four light sources are divided into left and right sides and arranged at the left and right sides two by two along a rotation direction of the polygon mirror, and separately eject laser light based on image signals of respective colors color-separated.
The laser light ejected from two light sources at the right side is reflected by the same reflection surface of the polygon mirror for scanning in a main scanning direction through the rotation of the polygon mirror, and is separately guided to two corresponding photoconductive drums at the right side. The laser light ejected from two light sources at the left side is reflected by another same reflection surface of the polygon mirror for scanning in a main scanning direction through the rotation of the polygon mirror, and is separately guided to two other corresponding photoconductive drums at the left side. Furthermore, the two light sources at the left side and the two light sources at the right side make pre-deflection optical systems independent, and thus are arranged at mutually different angles along the rotation direction of the polygon mirror.
Part of the laser light of each color after being deflected by the polygon mirror is detected by each of four photo detectors at an end part of a scanning start side in order to align writing timing of the latent images for the respective photoconductive drums by light beams of the respective color. Then, on the basis of the detection results of the four photo detectors, a control section of the image forming apparatus controls the writing timing of the laser light of each color.
However, as stated above, if two groups of light sources divided into left and right sides are arranged at different angle positions along the rotation direction of the polygon mirror, it is easy to generate a difference in light quantity obtained by detecting the laser light of each of left and right two systems that is reflected by the same reflection surface of the polygon mirror with the photo detector.
For example, attention is paid to one group of light sources arranged at the downstream side of the rotation direction of the polygon mirror. The laser light from one light source arranged at the upstream side along the rotation direction of the polygon mirror is vignetted by the reflection surface at the scanning start side on the same reflection surface more easily than the laser light from the other light source arranged at the downstream side. Further, the laser light from the light source at the downstream side along the rotation direction of the polygon mirror is vignetted by the reflection surface at a scanning end side on the same reflection surface more easily than the laser light from the light source arranged at the upstream side. Therefore, it is easy to generate a difference in the detected light quantity by the photo detector at the end part of the scanning start side on the reflection surface.
In order to control the writing timing of the laser light of each color with high accuracy, it is desired to uniformly align spot diameters and light quantity of the laser light detected by the photo detector. Therefore, in the conventional image forming apparatus described above, the width of each reflection surface of the polygon mirror becomes extremely large so that the laser light of each color entering the photo detector is not vignetted on the reflection surface at the scanning start end on the same reflection surface of the polygon mirror.
Conventionally, since a polygon mirror having six reflection surfaces is used in general, even if the width of each reflection surface is enlarged, the size of the polygon mirror can be confined in a specified size. However, in a case in which a polygon mirror having seven or more surfaces is adopted in order to achieve higher speeds, the size of the polygon mirror is larger than the specified size if the width of the surface is enlarged in the same way. In this case, the size of the apparatus is undesirably increased, inertia of the polygon mirror becomes large, time is necessarily spent until the rotation speed of the polygon mirror is stabilized, and as a result the first copy becomes slow.